That's what ours did when they started to die. First they just didn't open as much, then the mantles started to recede, then I could see the inside of the shell at the tops, then they stopped closing completely, then I knew they were dead and pulled them out. They can supremly foul the water if they die. As long as they still react to touch they are alive. How high up in the tank is it? and are you feeding it? We weren't feeding ours because they were over 4 inches long but I think ours starved to death anyway.